The costs of electronic data storage have been dramatically reduced as the storage densities on recording media have increased. This trend is particularly evident in hard disk drive technology. A conventional magnetic recording disk 10 for use in hard disk drives is depicted in FIG. 1. A cross section A of FIG. 1 is enlarged and depicted schematically in FIG. 2A. A typical recording disk 10 includes an aluminum (Al) substrate 12 covered by a nickel-phosphorous (NiP) plating layer 14. A chromium (Cr) underlayer 16 is provided on the nickel phosphorous plating. A magnetic material such as nickel, cobalt (Co), or a magnetic alloy is electroplated or sputtered onto the chromium underlayer 16 to form a continuous magnetic layer 18. A carbon overcoat 20 is deposited on top of the magnetic layer 18 and serves to protect the magnetic layer 18.
An enlarged top view of section B of the recording disk 10 of FIG. 1 is depicted in FIG. 2B. This view is not a physical view, but rather one provided by a magnetic force microscope following writing of data onto the disk 10. As recorded by a write head, the bits are substantially rectangular in shape and arranged in concentric tracks. In the illustrated example, a track width is approximately 2,000 nm. A small separation exists between the bits within a track as well as between bits of radially adjacent tracks.
The approximate shape and dimensions of a bit of a conventional longitudinally recorded magnetic bit are provided in FIG. 5a. The length of the bit is approximately 2,000 nm, the width of the bit is approximately 150 nm, and the depth of the bit is approximately 15 nm. A magnetic disk 10 that has been formed with a continuous magnetic layer 18 as depicted in FIG. 2A with the bit size described above has a recording density of approximately 1.7 Gbit/in.sup.2.
Increases in the areal density of magnetic storage media have been driven by the downward rescaling of hard drive assemblies. This resealing includes reducing the size of the grains making up the magnetic layer. In longitudinal recording, each bit is composed of numerous grains in order to maintain an adequate signal-to-noise ratio. However, reducing the grain size in order to reach higher storage densities is limited by the superparamagnetic limit. This limit occurs at the grain size at which thermal energy alone can trigger random magnetic switching of the grains.
A technology has been proposed to greatly increase the recording density of a magnetic disk by using discrete, single-domain magnetic elements embedded in a non-magnetic material. As proposed in Ultra High-Density Recording Storing Data in Nanostructures, Stephen Chou, Data Storage, September/October 1995 (pages 35-40), thin-film magnetic media are replaced by media that include discrete magnetic elements embedded in a non-magnetic disk. A corresponding cross-section A is depicted in FIG. 3A for a magnetic disk 10 having the proposed quantum magnetic structure. A silicon substrate 30 is covered by a plating base layer 32. A silicon dioxide layer 34 is provided on the plating base layer. The silicon dioxide forms a non-magnetic isolation layer in which magnetic columns are provided. The non-magnetic layer 34 has a depth of approximately 100 nm. Magnetic columns 38, approximately 50 nm in diameter, are provided in a vertical orientation in the non-magnetic layer 34. The magnetic columns 38 may be made of nickel or cobalt, for example. The non-magnetic layer 34 and the magnetic columns 38 are protected by an overcoat layer 36.
A schematic top sectional view of the proposed quantum magnetic disk is depicted in FIG. 3B, without the overcoat layer 36, to illustrate the arrangement of magnetic columns 38. In contrast to the magnetic force view of FIG. 2B, the view in FIG. 3B is a physical view. The centers of the magnetic columns 38 are separated by a distance of approximately 100 nm and are arranged in a grid-like manner. Each of the magnetic columns 38 represents a single bit for magnetic recording. The size of the bits (approximately 50 nm diameter) and the center-to-center separation of the columns (approximately 100 nm) produces a recording density of approximately 65 Gbit/in.sup.2.
The costs associated with achieving such a large storage density are prohibitive as the proposed manufacture of quantum magnetic disks utilizes expensive semiconductor processing techniques. An exemplary fabrication process was described in Chuo as including electron beam lithography to define the size and location of each bit in the disk. After development and chrome etching, a reactive ion etching step is performed to create a silicon dioxide template with column openings. Nickel or another electromagnetic material is then electroplated into the column openings to form the magnetic columns. The disk is then polished to planarize its surface.
In addition to the greatly increased costs of manufacture of the disks, the proposed quantum magnetic disk requires complicated non-Winchester recording technology not currently available. Hence, although providing a very high recording density, the proposed magnetic disk remains an impractical alternative to conventional magnetic recording media.